1. Field of the Invention
The present invention relates to a phase-locked loop (PLL) circuit which generates a clock signal synchronized with a color burst signal contained in a composite color picture signal (composite picture signal).
The present invention further relates to a phase-locked loop circuit which is provided on a receiver side of a digital transmission system, and which regenerates a clock signal which is used on a transmitter side, based on information on the frequency of the clock signal. In particular, the present invention relates to a phase-locked loop circuit which is provided on a receiver side of a picture signal transmitting system, and regenerates a sampling clock signal which is used on a transmitter side for sampling an original picture signal to generate the (sampled) picture signal to be transmitted, where a transmission clock signal used for transmitting the picture signal does not synchronize with the sampling clock signal.
2. Description of the Related Art
In a picture signal transmitting system transmitting a composite color picture signal, high quality is required for a composite color picture signal which is regenerated on a receiver side from a received picture signal. In particular, the precision of frequencies and vector stability of color signals depend on the precision of a clock signal which is used for regenerating a composite color picture signal. Such a clock signal is generated as a clock signal which synchronizes with a color burst signal contained in the composite color picture signal.
According to one of the conventional methods of regenerating a clock signal which synchronizes with a color burst signal contained in the composite color picture signal, first, the color burst signal is extracted by generating, based on the timing of the horizontal synchronizing signal, a gate pulse which is active at the timing of the color burst signal in the composite color picture signal, and extracting the color burst signal by using the gate pulse. Then, the extracted color burst signal is used as a reference signal in a phase-locked loop circuit which generates the clock signal. In the phase-locked loop circuit, the phase of the generated clock signal is compared with the phase of the extracted color burst signal for each timing of the horizontal synchronizing signal to control the phase of the clock signal so that the clock signal synchronizes with the color burst signal.
However, no color burst signal exists for the time of a vertical blanking signal contained in the composite color picture signal. Therefore, the frequency of the clock signal may fluctuate in the duration of a vertical blanking signal due to the absence of the color burst signal as a reference signal. Conventionally, a second phase-locked loop circuit is provided in the stage following the above phase-locked loop circuit to reduce the fluctuation of the frequency of the clock signal. However, since the period of the variation of the frequency is long, the comparison frequency (the frequency of phase comparison performed) in the second phase-locked loop circuit must be low. The low comparison frequency in the second phase-locked loop circuit results in a low loop gain in the second phase-locked loop circuit, and the pulling-in operation at a transitional period becomes slow due to the low loop gain.
When a transmission clock signal used for transmitting sampled information does not synchronize with a sampling clock signal which is used on a transmitter side for sampling certain information to generate the (sampled) information to be transmitted, it is required to regenerate, on a receiver side, the sampling clock signal synchronized with the transmitted information, and the regenerated sampling clock signal must have a precise frequency.
For example, when a transmission clock signal used for transmitting a (sampled) picture signal does not synchronize with a sampling clock signal which is used on a transmitter side for sampling an original picture signal to generate the (sampled) picture signal to be transmitted, it is required to regenerate on a receiver side the sampling clock signal synchronized with the transmitted picture signal, where the regenerated sampling clock signal must have a precise frequency.
According to one of the conventional methods of regenerating on a receiver side a clock signal synchronized with a picture signal sampled on the transmitter side in a picture transmission system in which a transmission clock signal used for transmitting the picture signal does not synchronize with a sampling clock signal used for sampling an original picture signal to generate the (sampled) picture signal to be transmitted, first information on a first difference between frequencies of the sampling clock signal and the transmission clock signal is generated on the transmitter side, and is transmitted together with the picture signal to the receiver side. Then, in a phase-locked loop circuit on the receiver side, second information on a second difference between frequencies of a regenerated sampling clock signal which is regenerated by the phase-locked loop circuit and the transmission clock signal is generated in the same manner as the manner in which the first information is generated on the transmitter side, and a third difference between the first and second differences is obtained to control a voltage controlled oscillator in the phase-locked loop circuit based on the third difference so that a clock signal which synchronizes with the picture signal is regenerated as the sampling clock signal.
However, the conventional phase-locked loop circuit which is to be used on a receiver side of a picture transmission system in which a transmission clock signal used for transmitting a picture signal does not synchronize with a sampling clock signal which is used on a transmitter side for sampling an original picture signal to generate the (sampled) picture signal to be transmitted, has the following drawbacks.
(1) Firstly, the frequency of the regenerated sampling clock signal rapidly varies at a transitional period such as the time of power-on or the stop of the transmission of the picture signal, and therefore the color signals which are regenerated from the composite color picture signal by using the regenerated sampling clock signal rapidly vary. This may cause irregular chromaticity in the regenerated color picture.
(2) Secondly, the frequency of the regenerated sampling clock signal greatly swings back and forth before converging on the frequency of the original sampling clock signal, for example, at a transitional period such as the time of power-on or the stop of the reception of the picture signal.
(3) Thirdly, the converging time of the regenerated sampling clock signal is long.
(4) Fourthly, the frequency of the regenerated sampling clock signal regenerated on the receiver side may greatly vary when the transmission of the picture signal is stopped or restarted. This may cause irregular chromaticity in the regenerated color picture at a transitional period caused by the stop or restart of the picture signal.